Conventionally, in producing the same product in a large quantity, it is common to perform a series of processes on a production line. Such a production line is called a “conveyer production line” and is widely adopted in manufacturing products such as automobiles. In the conveyor line method, it is important to keep a processing time required for each process included in the production line constant. This is because if one process takes a longer processing time compared to other processes, this process becomes a bottleneck and reduces the operation rate of processing apparatus used in other processes.
On the other hand, in a production line of semiconductor chips or the like, the same processing apparatus can be commonly used in more than one process, unlike the conveyor line method. Such a production line is called a “job-shop production line”. For example, in semiconductor chip manufacturing, processes such as wafer cleaning, film formation of conductive material or a dielectric material, photolithography, and etching are repeated many times.
Among these processes, in the wafer cleaning, a number of wafers can be collectively processed even if the same processing apparatus (a cleaning apparatus) is commonly used in more than one process. Therefore, there are few cases where this process becomes a bottleneck. In the film formation process and the etching process, it is often difficult to commonly use the same processing apparatus (a film-formation chamber and an etching chamber) in more than one process since conditions such as process gases are different depending on materials to be formed into a film or to be etched. With this reason, in the film formation process and the etching process, a chamber specially prepared for each process is often used. Therefore, there are few cases where these processes become a bottleneck either. However, the film formation process and the etching process may be a bottleneck due to failure of the apparatus.
In contrast to this, since a stepper is considerably expensive, the same stepper is commonly used in more than one photolithography process generally. In addition, unlike the cleaning process and the like, it is impossible to collectively process a number of wafers, and required to process wafers one by one. Therefore, in production lines of semiconductor chips, the photolithography process is the bottleneck in most of the cases.
In the job-shop production line, if a key process (the photolithography process in the above example) in which the same processing apparatus is commonly used in more than one process becomes a bottleneck, it can be very difficult to decide which in-process product should be processed in priority among in-process products accumulated before the bottleneck apparatus (the stepper in the above example).
FIG. 14A is a schematic diagram showing processing apparatuses used in one example of a job-shop production line. FIG. 14B is a table showing processing apparatuses used in respective processes shown in FIG. 14 and the number of in-process products that are accumulated therebefore.
As shown in FIG. 14A, this production line is composed of four processing apparatuses A, B, C, and D, and a process performed with the processing apparatus B is the key process among these processes. Specifically, six processes, process 1 to process 6, are performed in this production line as shown in FIG. 14B, and the processing apparatus B is used in process 2, process 4, and process 6.
In process 2, process 4, and process 6 in which the same processing apparatus B is used, three production lots, four production lots, and two production lots of in-process products are accumulated respectively as shown in FIG. 14B. In this case, the processing apparatus B is required to make some determination on which of the in-process products should be processed in priority. Although it is generally determined that the process (process 4 in this example) having the most in-process products should take priority, such simple determination is not necessarily appropriate in an actual production line.
On the other hand, as a method of determining priority orders in the job-shop production line, a determining method described in Japanese Patent Application Laid-open No. H11-145021 is known. In the technique disclosed in the Japanese Patent Application Laid-open No. H11-145021, however, the priority orders are determined merely based on a processing time required in a process subsequent to a key process to be the bottleneck; therefore, by this method, it is impossible to ease accumulation of many in-process products at the key process, to equalize the number of in-process products at respective processes, nor to prevent reduction of the operation rate of the processing apparatuses. Further, in a production line of semiconductor chips, CONWIP (CONstant Work-In-Process) in which manufacturing of new products is started for an amount equivalent to the number of finished products is often adopted. Therefore, by the method described in the Japanese Patent Application Laid-open No. H11-145021, the priority orders cannot be properly determined in consideration to the CONWIP.